This invention is concerned with acoustic techniques for measuring the dynamic viscosity of a material, and particularly with the measurement of the viscosity of a composite material during the curing process for the composite.
A composite material typically includes a base or substrate material, such as a thermally cured polymer ior epoxy resin, the substrate being strengthened by the addition of a fibrous component such as carbon, graphite, boron, or nylon. Composites exhibit extremely high strength-to-weight ratios in comparison to other structural materials. As a result, their use is becoming increasingly important in applications which require high strength as well as light weight, such as the manufacture of aerospace vehicles.
In fabricating a structure with composite materials, a part is manufactured by first positioning layers of raw or partially cured composite materials in a mold. When the desired shape has been built up, the part is subjected toi a curing process in a pressurized oven known as an autoclave. Under the influence of an elevated temperature in the autoclave, the polymer molecules of the resin grow into longer chains and branches, and cross links between the chains are formed. In this manner, the polymerization of the resin substrate is completed, causing the molded shape to become permanent and leaving the composite material hard and durable.
The composite production process may be usefully characterized by continuously measuring the structural parameters of the composite during the solidification of the material. One important parameter of the curing material which is influenced by the rate of the curing reaction is the viscosity of the resin substrate. Knowledge of the changing viscosity can be used, for example, to minimize porosity in the final product. Measuring the process, however, presents some difficult problems. The rate at which the curing reaction proceeds is a complex function of temperature and pressure which depends, inter alia, upon the thickness and geometry of the part being fabricated, the thermal equilibrium between the part and the mold, the temperature of the environment around the part, and the thermal mass of the autoclave. At times during the cure, the viscosity will be low enough to allow the resin to flow. Although a controlled flow of the resin may be desirable to achieve the required thickness or strength in the part, if the resin is allowed to flow upredictably, microvoids or variations in the thickness of the part can result. Consequently, control of the resin viscosity is an important aspect of the cure process.
For any given part geometry, the flow of the resin is determined by its viscosity and ambient pressure. The viscosity of the resin is, in turn, a function of temperature and of the time the resin has been subjected to the final curing process. Thus, the temperature and pressure can be varied during the cure cycle to control changes in the viscosity. For a particular resin and a given variation in temperature during the cure cycle, it is theoretically possible to predict the viscosity of the resin at any time during the cure cycle. Consequently, process control technicues in the prior art have involved monitoring the temperature and pressure during the cure cycle and adjusting these parameters, either manually or by computer control, in an attempt to maintain the viscosity of the composite at the theoretical ideal.
It has been found, however, that actual variations in the viscosity of the resin during the cure cycle frequently do not match the predicted viscosity profile. This is due to a number of factors, such as variations in the moisture content of the resin or disparities in the production techniques used to make the pre-impregnated resin. Furthermore, the polymerization and cross-linking reactions of the resin, which ideally occur only during the final cure step, also proceed, albeit at a slower rate, in the pre-impregnated resin. Thus, even resins having the same original chemical composition may exhibit different states of polymerization when the final curing process is initiated and will therefore display somewhat different viscosity profiles under the same cure conditions. Consequently, it is desirable to monitor temperature, pressure, and, most important, viscosity at various locations on the part and at various times during the cure cycle, and to adjust the applied temperature and pressure in accordance with the disparity between the desired temperature, pressure, and viscosity profiles and the measured values.
Although mechanical measurements of viscosity have been made in the prior art, a mechanical approach requires the insertion of a probe into the measured component. This is generally not feasible during the manufacture of a composite part, especially when the viscosity must be measured at different times during the cure cycle and at numerous locations within the composite part. Chemical techniques, such as high performance liquid chromatography, differential scanning calorimetry, and infrared spectroscopy, are also known in the art for monitoring the cure state of a resin. These techniques, however, are difficult to implement in a manufacturing environment. Another method which has been used to measure the viscosity is to relate it to changes in the substrate's dielectric properties. This method, however, suffers from a lack of reliability and low signal-to-noise ratios.
A variety of acoustic testing methods have alsio been employed to measure resin viscosity. A class viscometer, such as the torsional torque viscometer, mechanically measures the force required to turn a vane or propeller-like structure inserted into the test liquid. Using ultrasound to measure viscosity implies that the viscosity is measured at high frequencies. The viscosity so measured, which is known as the dynamic viscosity, is much lower in value than the viscosity usually measured with a classical viscometer, because the high frequencies involved require more rapid motions in the viscous medium than are associated with relaxations in the medium. Nevertheless, the behavior of viscosity as a function of temperature or pressure has been observed to be similar at high and low frequencies. Consequently, the dynamic viscosity is a useful parameter for describing the state of the viscous medium. In addition, a potential advantage of using the propagation of ultrasonic waves to measure the viscosity of a medium is that the wave propagation depends directly on the mechanical constants of the medium of propagation. Ultrasonic data analysis techniques provide algorithms for deriving these mechanical constants from the measured propagation characteristics. The usual acoustic technique of measuring the attenuation of longitudinal waves propagating in the viscous medium requires many assumptions and many conditions to be fulfilled that limit its applicability. All the other causes of ultrasonic attenuation, for example, such as diffraction, dispersion, and thermoelastic loss, must be negligible in comparison to the viscous loss. In addition, the sum of the volume and shear viscosities is measured, rather than only the shear viscosity. Furthermore, in longitudinal wave techniques the ratio of the imaginary part of the bulk modulus to the real part must usually be assumed to be much lower than unity. The viscous medium must have sufficient thickness that the different echoes in the pulse-echo train can be resolved, yet be thin enough that the first echoes are detectable, and internal reflections inside the composite laminates must be assumed to be negligible. Other ultrasonic methods known in the prior art for measuring viscosity use the reflection of plane shear waves, resonance techniques, or guided travelling waves.
One method used to measure the dynamic viscosity of a medium is to first launch a pulse in a solid (a buffer rod) which is not in contact with the viscous medium. This measurement provides a reference waveform for the vibration characteristics of the buffer rod alone. Another pulse is generated after the buffer rod is placed in contact with the viscous medium. The received waveform is then compared with the reference waveform. From the results of these twio measurements, the viscosity can readily be deduced. By decinvolution, the reference signal allows unwanted information, such as transducer frequency characteristics, changes of velocity and attenuation of the buffer material with temperature, and changes of buffer length with temperature, to be removed. A double buffer with a partially reflecting interface has also been used to provide the reference. In the latter arrangement, the time domain is used to separate the reference signal from the unknown resin signal. This requires the use of short pulses which can be time-resolved sufficiently to give them separate treatments in signal processing.
This method for measuring the dynamic viscosity suffers from two disadvantages, both related to the fact that the change of phase due to the presence of the viscous medium is extremely small. The bond between the piezoelectric element and the buffer rod must be the same for both measurements, and the length of the buffer rod must not change. When variations in temperature or pressure occur, these two conditions may be hard to fulfill.
Thus the acoustic methods known in the prior art are limited in their ability to monitor viscosity at high temperatures, cannot measure high values of viscosity, and suffer from inaccuracies introduced by such factors as temperature instability, stray capacitance, and unreliable bonds between portions of the acoustic apparatus. In particular, the acoustic techniques known in the prior art require either partial or total immersion of the testing apparatus in the liquid whose viscosity is to be measured. This requirement is unacceptable in such applications as the composite curing environment, where the structural integrity of the curing part could thereby be adversely affected.